(D) Carbon trading is a mechanism under Kyoto protocol which allows nations who are unable to meet their reduction targets to purchase carbon credits under a unified regulatory framework. The Montreal Protocol is an international treaty signed in 1987, designed to protect the ozone layer by phasing out the production of numerous substances that are responsible for ozone depletion. Agenda 21 is a non-binding action plan of the United Nations with regard to sustainable development. It is a product of the Earth Summit (UN Conference on Environment and Development) held in Rio deJaneiro, Brazil, in 1992. Earth Summit + 5 was held in 1997 in New York to review and accelerate the implementation of Agenda 21 and other agreements made in Rio.